In the electric/electronic, transportation, and other fields, more than ever electronic components or parts are now incorporated for precise control of energy consumption. In the transportation field, for example, the transition from gasoline-powered vehicles to hybrid vehicles, plug-in hybrid vehicles, and electric vehicles poses a need to mount electronic components or parts such as motors, inverters, and batteries, which have been unnecessary for the gasoline-powered vehicles. Also in the body-related mechanisms such as engine control, power train, and air conditioner control mechanisms, more advanced control functions are required and more control systems are necessary. Thus, the number of electronic control units (ECUs) loaded is increasing every year. The number of electronic components built in such ECUs is also increasing. Accordingly, heat-conductive silicone compositions are now indispensable to remove heat from these heat-generating electronic components or parts and conduct the heat to cooling units efficiently.
Since a larger number of electronic components or parts must be recently installed in a limited space, they are placed under widely varying conditions (e.g., temperature and mount angle). For example, in many cases, heat-generating electronic components or parts and heatsinks are not placed horizontally, and heat-conductive materials connecting them are mounted in certain inclination. In such service environment, a one-part addition type heat-conductive silicone composition is sometimes used in order to prevent the heat-conductive material from slipping and falling out of the space between the heat-generating parts and the heatsinks (Patent Document 1: JP 3580366). That is, the heat-conductive material, after heat curing, is prevented from slipping and falling out of the space between the heat-generating parts and the heatsinks. As a result, heat dissipating properties are maintained for a long term. However, this one-part addition type heat-conductive silicone composition suffers from several problems. For example, the heat-conductive silicone composition must be refrigerated or frozen during storage and thawed prior to use. When the one-part addition type heat-conductive silicone composition is applied in place, it must be heated and cooled. Thus the manufacturing system using the material must be equipped with a heating/cooling oven. The heating and cooling steps take a long time, leading to a reduction of manufacturing efficiency. From the standpoint of energy efficiency, the heating step is not considered efficient because not only the heat-conductive material, but also the applied part must be heated in entirety.
There is also a problem that the heat-conductive material is under-cured if metal cutting fluid containing a curing inhibitor such as an amine compound is left on the coating surface. It is an additional problem that the heat-conductive material increases its hardness with the lapse of time due to excessive addition reaction by the heat release from heat-generating parts, which causes stresses to the installed component.
To save time and labor for storage/thaw management and heating/cooling steps in application of the one-part addition type heat-conductive silicone composition and to alleviate concerns about cure inhibition, a one-part addition type heat-conductive material which has been heat-crosslinked during its preparation is already proposed (Patent Document 2: JP 4130091). This heat-conductive silicone grease composition has overcome the above-mentioned problems, but a new problem arises as the tradeoff that the material is too viscous to coat, and heavy loading of heat-conductive filler is difficult due to the high viscosity of the base polymer. The above-mentioned problem of hardness increase with time is left yet unsolved.
On the other hand, there are proposed condensation-curable heat-conductive silicone compositions which have a low viscosity at the initial, which are coated and cured or thickened with airborne moisture in a room temperature environment to prevent them from slipping and falling out of the space between the heat-generating parts and the heatsinks, and which allow for room temperature storage (Patent Documents 3 to 5: JP-A 2004-352947, JP 4787128, and JP 5733087). These materials are useful in that refrigeration or freezing is not required during storage, heating/cooling steps are not required in application, and manufacturing efficiency is high. However, these materials commonly have the drawback of inferior deep section cure because their curing system utilizes airborne moisture for curing. This results in the problem that a cured product of the composition experiences deterioration such as cracking and slipping upon application of thermal loads during heat exposure or thermal cycling, detracting from heat dissipating properties. In addition, since excessive condensation reaction proceeds with airborne moisture, the hardness increase with time is still the outstanding problem for this curing system.
For the purposes of preventing deterioration such as cracking and slipping and alleviating hardness increase with time in cured products of condensation-curable heat-conductive silicone compositions, an improvement in deep section cure is considered a common countermeasure. In condensation-curable heat-conductive silicone compositions, a catalyst is generally added to promote condensation curing. However, if large amounts of catalysts are added or catalysts having strong catalytic activity are used, with the extra intention to promote deep section cure, then the composition may be considerably exacerbated in shelf stability and handling. It is desired to establish a condensation-curing system which is effective for improving deep section cure without sacrificing shelf stability and handling.